Electronic structure characterization and bandgap engineeringofsolar hydrogen materials Page: 4 of 9
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The difference in structural arrangement of these carbon forms gives rise to the wide differences in their physical
properties. Carbon has an atomic number of 6 and has the 1s22s22p2 configuration in its electronic ground states. The sp-
orbital, and the sp3-hybrid orbital indicate rotational symmetry. Bonds of this kind are call 7t-bonds. The electrons
involved in such bonds are called 7t-electrons. With double bonds, so-called a-orbitals occur with corresponding a -
electrons. Such orbitals are not symmetrical with regard to their bonding orientation.
The normal carbon K-edge x-ray and absorption and emission spectra of C60, HOPG, and diamond presented in Figure 2,
where large differences in spectral profile are observed. The spectra of diamond and graphitic carbon show a wide band
with some shoulder structures, where the energy positions of the peak maximum and band shapes are largely different.
In some studies related to vapor deposition the X-ray emission spectral profile has been used as a means to identify
certain chemical states [16]. The X-ray absorption spectrum of diamond shows no 7t* contribution, and the absorption
feature at 288.9 eV is the diamond exciton. In contrast, the spectra of C60 exhibit clearly resolved emission peaks,
indicating strong molecular character in their solid phase. The marked band edges show the large bandgap in diamond in
contrast to metallic behavior of graphite.
5. BANDGAP AND DOPING
The electronic structure of insulator YBa2Cu306.02 and superconductors YBa2Cu30.94 was studied with a combination of
XAS and XES [17]. The O is absorption spectra of YBa2Cu306.02 and YBa2Cu30.94, which reflect the unoccupied O 2p
states, are shown in Figure 3. The absorption prepeak in YBa2Cu306.02 starting from 530 eV is attributed to the covalent
mixing states of O 2p-Cu 3d. The XAS spectrum of YBa2Cu30.94 shows a strong increase of the intensity at the lower
binding energy region. This behavior is similar to that in the O is absorption edges of other cuprates [18,19], which is
due to the doping-induced shift of the Fermi level into the valence band, the creation of the holes in the valence band,
and a transfer of spectral weight from states in the upper Hubbard band to the upper edge of the valence band..6
1
c,Valence Iand Conductio
YBa
* YBa520
525
530
2Cu3O6)2
535
Energy (eV)
Figure 3. Resonantly (534.7 eV) and normal (545.5 eV) excited XES spectra of liquid water in comparison with
calculations for SYM, A-ASYM and D-ASYM species. Excitation energies are marked in the XAS spectrum.I I i I I i i i I I i I I I I I i i
n Band..-
1Cu306.94
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Guo, Jinghua. Electronic structure characterization and bandgap engineeringofsolar hydrogen materials, article, November 1, 2007; (https://digital.library.unt.edu/ark:/67531/metadc880814/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.